This invention relates to powder metallurgy and particularly to the preparation of substantially fully dense articles by sintering.
U.S. Pat. No. 3,700,435 relates to a hot isostatic pressing process for consolidation of powder metals. In that process, the powder metal is charged to a mold and the mold is placed in a container. The remainder of the container is then filled with a secondary pressure media. The entire assembly is heated and pressurized. The function of the secondary pressure media is to transfer pressure applied to the outer walls of the container to the mold. The interior of the container, including the secondary pressure media, is filled with an inert gas for the heating cycle and evacuated prior to pressing. U.S. Pat. No. 3,700,435 is characteristic of the many hot isostatic pressing processes used for powder metal consolidation all of which require the use of presses and pressure vessels which are extremely expensive.
U.S. Pat. No. 3,704,508 discloses a process for consolidating certain alloys in which no pressing or hot working is necessary. The patent describes pretreating the powder metals with an electron donor compound and subsequently applying heat and vacuum to activate the powder surfaces prior to sintering. The patent describes a method by which high density parts can be produced by sintering metallic powder in a glass mold. These molds must be supported in some manner as the glass becomes relatively fluid at the metal sintering temperatures. Furthermore, the support container must be of the general shape of the glass mold to maintain the shape of the mold and the sintering mass. As this process is applicable to a wide variety of shapes, each requiring a different container configuration, a large number of supporting containers are needed. After placing the glass mold in a support container, usually a carbon container, it is covered with borosilicate glass chips. The glass chips provide support for the compact during sintering, for as the glass softens, it flows over the mold filling the voids and prevents the mold from shifting within the carbon container.
There are several major disadvantages inherent in the process described in U.S. Pat. No. 3,704,508. The most formidable problem, already mentioned, stems from the large number of different glass molds being used. Each mold requires its own carbon or graphite container. It has also been found that the rapid oxidation of the carbon or graphite mold during consolidation at, for example, 2100.degree. F. results in a short life for the container. Protective atmospheres, such as argon or nitrogen, are used to extend the container life. This is helpful since the cost of machining the carbon or graphite containers for complex shapes is very expensive. Even so, with the protective atmosphere, the container life is limited requiring the manufacturer using the process of the aforesaid patent to maintain several spare containers for producing a given shape. Finally, discharging the sintered metal compact from the container and mold is quite difficult due to the flow of glass into the machining marks and other faults in the container.
We have now discovered a substantial improvement in powder metal sintering processes such as those disclosed in U.S. Pat. No. 3,704,508. It is no longer necessary to make expensive carbon or graphite containers and to protect them during sintering with a protective atmosphere and then to discard them anyway due to change of shape resulting from oxidation after several uses. It is an advantage according to this invention that containers machined to near the shape of the glass mold are not required nor is it necessary to use graphite containers at all. It is an advantage according to this invention that heat can be transferred to the glass mold containing the powder to be consolidated, at a substantially uniform temperature over the entire surface of the glass mold. It is yet another advantage that no special atmospheres are required for the practice of this invention nor is expensive hot isostatic pressing equipment required.